1. Field of the Invention
The present invention relates to a ridge-type semiconductor laser devise to be used in an optical disk device such as a CD-R/RW drive or DVD-RAM drive, and to a fabrication method thereof.
2. Description of the Related Art
Attempts have been made to increase the recording speed of optical recording devices. For example, CD-R drives having a so-called “16 speed” recording speed are already being put to practical use. Accordingly, in optical recording devices with a high recording speed, an instantaneous start up of high output laser light is required. There are ridge-type semiconductor laser devises that use compound semiconductors, these being laser devises that satisfy such a required characteristic.
FIG. 3 is a schematic cross-sectional view of the structure of a conventional ridge-type semiconductor laser devise, showing a cross section that lies orthogonal to the longitudinal direction of the devise.
A lower cladding layer 32, an active layer 33, and a first upper cladding layer 34 are sequentially stacked on a substrate 31. A ridge-shaped second upper cladding layer 37 is formed on the first upper cladding layer 34 in a position substantially central in the width direction of the devise, so as to extend in the longitudinal direction of the devise. Current blocking layers 36 are formed on both sides of the second upper cladding layer 37. A contact layer 38, which makes ohmic contact with the second upper cladding layer 37, is formed on the second upper cladding layer 37 and the current blocking layers 36.
An n-side electrode and a p-side electrode (not illustrated) are formed on the lower face of the substrate 31 and the upper face of the contact layer 38 respectively.
The conductivity type of each portion is as follows. For example, the substrate 31, lower cladding layer 32 and current blocking layers 36 are n-type layers, and the first upper cladding layer 34, second upper cladding layer 37, and contact layer 38 are p-type layers. The active layer 33 can also be an n-type layer or a p-type layer, or can be an undoped layer.
During laser light emission, light emission takes place upon recombination of electrons and holes in the active layer 33.
In the section above the active layer 33, because the conductivity type of the current blocking layers 36 alone differs from that of the other sections, current is confined between the current blocking layers 36. The injection of current from the second upper cladding layer 37 to the active layer 33 takes place via the bottom face of the second upper cladding layer 37. Accordingly, within the active layer 33, light emission occurs upon recombination of electrons and holes (carriers) in a region E3 (referred to as “light emitting region E3” hereinbelow) whose width is substantially equal to the width S3 of the bottom face of the second upper cladding layer 37 (referred to as “injection current width S3” hereinbelow). Here, carriers are not consumed uniformly within the light emitting region E3, there being greater consumption of carriers in the vicinity of the width direction center portion.
Therefore, even though carrier consumption is non-uniform, the diffusion of carriers (particularly a small number of carriers) results in the carrier concentration being to some extent uniform within the light emitting region E3. However, when the operating current is high and the injection current width S3 is wide, diffusion is unable to replenish carriers and the light intensity distribution is then accordingly non-uniform. In other words, the section within the light emitting region E3 which exhibits high intensity light emission moves over to the section in which the carrier concentration is high, which accordingly makes the light output unstable. A laser whose light output is unstable cannot be used in optical recording or optical reading applications. Thus, the maximum rated light output of a laser devise is established within a range in which the light output is not unstable.
Non-uniformity of the carrier concentration can be eliminated by narrowing the injection current width S3. That is, as a result of making the injection current width S3 equal to or less than the carrier diffusion distance (carrier diffusion length) in the active layer 33, a small number of carriers can be reliably replenished in the section where a large number of carriers is consumed, whereby it is possible to realize uniform light emission within the light emitting region E3. That is, by means of such a structure, the light output region within which a stable light output can be obtained can be made large, which makes it possible to raise the maximum rated light output of the laser devise.
Further, when the injection current width S3 is narrowed, the upper face width D3 of the second upper cladding layer 37 also narrows. This is because the inclined planes of the second upper cladding layer 37 are formed at fixed angles of inclination, meaning that when the thickness of the second upper cladding layer 37 is fixed, the injection current width S3 and the upper face width D3 cannot be changed independently of one another. Because the current flows through the interface between the contact layer 38 and the second upper cladding layer 37, a narrowing of the upper face width D3 causes shrinkage of the surface area of the interface between the contact layer 38 and the second upper cladding layer 37, which in turn raises the resistive value of the devise.